| Making compost |
HOME MAKING COMPOST
Eliminating the nuisance
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By Lynn McCracken Lucas A. Blending Every mason knows the most important element of every project is the cornerstone. Do a proper job of it, and the chances of a straight, level, plumb finished product are good. Muck it up, and nothing but demolition can correct it. Blending is the cornerstone of compost production. Get it right, and the process will hum merrily along without a whole lot of human intervention. Get it wrong, and the fix may be reprocessing. Blending must balance the many factors influencing the composting process to be successful. (1) CARBON AND NITROGEN SOURCES (C:N RATIOS). In the universal language of composters, the colors brown and green refer to carbon-to-nitrogen ratios. So much brown stuff, plus so much green stuff, provides the balance the process needs. The ideal ratio at the start of processing is 30 parts carbon (brown) to one part nitrogen (green) or a C:N ratio of 30:1. A range from 25:1 to 35:1 is considered optimum. The composting process reduces the C:N ratio, and finished compost will typically have a C:N ratio around 10:1. However, composting isn’t like getting dressed in the morning. One doesn’t actually go out to the feedstock pile and color coordinate. As a general rule, green vegetation has higher nitrogen content than brown vegetation. But sawdust, straw, and rice hulls are carbon sources ... not always brown. Freshly mowed grass, potato peels, humanure … these are nitrogen sources, and not all of them are green. So forget about actual color and look at composting mix-n-match from a microbe’s perspective. Nitrogenous materials give them quick energy; carbon sustains them over the long-term. There’s also a relationship between C:N ratios and moisture content. Generally, high carbon materials are drier than low carbon feedstocks. An analysis of the waste to be composted will reveal the C:N ratio of the material. Such tests are typically inexpensive when processed by state soil labs through the local Cooperative Extension office. While not absolutely necessary for the backyard, successful commercial and municipal operations pull a sample of every waste stream for analysis. Where a material falls above or below the “ideal” of 30:1[1] determines its relative value as a carbon or nitrogen source. But keep in mind that C:N ratios are based on weight, not volume, and it’s # pounds of carbon to # pounds of nitrogen … not the weight of the material. To calculate the C:N value of any feedstock mix, one can add total pounds[2] C and N (based on a waste analysis) or percentages[3] of carbon and nitrogen. But it’s easier to use a web-based compost mix calculator.[4] They’re fast, free, and do not require any mathematical skills. (2) MOISTURE. Like Goldilocks, compost-makers strive for moisture levels that are not too low, not too high, but just right. Ranges of 45 to 65 percent moisture are ideal at the start of the compost process and will facilitate rapid biodegradation. Finished compost will be in the range of 30-70 percent. Too much moisture inhibits oxygen flow and encourages anaerobic organisms (bad bugs). Too little moisture impedes the free movement of bacteria (good bugs) throughout the composting mass. It’s possible to purchase moisture meters for under $10, over $1,000, and all points in between. Many moisture meters don’t have the range needed for composting, so be sure to check before buying one. But a meter is not a must-have item. The time-tested “squeeze” method is one that works for composting operations of any size:
If there’s a problem with the moisture level of the blended admixture, fix it. This is a critical factor in successful composting. (3) POROSITY, PARTICLE SIZE, AND HOMOGENEITY. Adequate pore space is essential for the movement of air and water throughout the composting mass. Air not only delivers oxygen, but also removes excess heat, while water allows microbes to move freely. In order to create and maintain adequate pore space, feedstocks should be blended to a uniform particle size (.5 to 2 inches is recommended … go low if using a forced aeration system) and the mix must be homogeneous. The varying constituents should be evenly distributed throughout the pile and have a uniform appearance free of clumps or pockets of any single feedstock. Homogeneity also insures even temperature dispersion for pathogen kill. B. Processing and curing. There is a relationship between the size and shape of the compost pile and its ability to reach and maintain desired temperatures. Though the mechanism is not fully understood, it is believed to work because the pile is large enough to both generate sufficient heat and become self-insulating. The six-foot high, conical shape of a well-constructed compost pile (elongated if in a windrow), repels rain and allows the pile to form a __ to ___-inch crust, improving heat retention. Most backyard piles do not have enough mass for this mechanism to kick in, which is why toxic and pathogen-laden feedstocks are to be avoided by home composters. Within a pile that has been properly blended and constructed, temperatures will begin to climb almost immediately, reaching the 131°F mark for pathogen kill for static forced aeration and in-vessel composting within the first 24-36 hours of placement. Inherent in these systems are mechanisms to control and moderate temperatures, keeping them within ideal ranges for regulatory compliance. In windrow operations, the temperature keeps climbing, gets too hot, and kills the microbes. The process cools down and must restart itself, repeating this cycle many times to achieve the same level of biodegradation and pathogen destruction as other methods. It is not the windrow itself, but the absence of process control, that dictates longer processing times. At the end of the active phase, when the original feedstocks are no longer discernable and the composting mass has met all requirements for PFRP and VAR, the microbes will calm down as the process enters a curing phase of several months while the compost matures. While the exact time requirement varies, depending on the feedstocks and the level of active management, a minimum of six months is typical, during which time the last of the carbon breaks down, the odor changes from prickly to the rich aroma of humus, and the color turns dark. Microbes are still active, but have settled into a barely perceptible hum. C. Tools and equipment. As with all things in life, using the proper tool for the job makes composting much easier. (1) PEOPLE-POWERED. In the backyard, a sharp shovel may be all that’s needed to chop and blend feedstocks and turn them during processing. This versatile tool can even be used to toss finished compost into the air so it breaks into smaller particles on impact with the garden or landscaping bed. But composting quickly moves beyond the time and capability limitations of the shovel as the size of the composting operation increases. Those with the time and energy to make compost the old-fashioned way can add a few more hand tools to make the process more efficient. Feedstocks still need to be reduced to a uniform particle size, and the shovel may still be the best tool for that particular job, but inexpensive compost turners and tumblers make the blending and biodegradation phase much easier. These devices may be as simple as a long handle with wings or forks on the end that are driven into the pile and lifted to mix things up or consist of drums that are turned by a crank or rolled on the ground. (2) ANIMAL-POWERED. Between human-powered and fuel-powered stands the four-legged creatures who partnered in almost every human endeavor until the invention of the steam engine in 1721. (3) EARTH-POWERED. For most, equipment powered by earth’s natural resources (coal, oil, water, sunshine, wind) converted into electricity or a petroleum product will be used to produce compost.. Chipping and shredding. Tub grinders and trommel screens are needed for industrial operations, capable of reducing just about any organic (and some non-organics like gypsum products) to a uniform particle size. A tub grinder does the job by using a hammermill to shred the material and a grate at the bottom that allows only pieces of a certain size to fall through. A trommel screen is a large cylinder lined with a metal mesh. Materials tumble and break apart as the trommel rotates, and the finer pieces fall through the screen and onto a conveyor. The larger pieces eventually work their out to the end of the unit, where they fall onto another conveyor for additional or re-processing. Particle size is controlled by the speed of rotation, as well as screen size.. Tub grinders are faster than trommels, but they don’t separate contaminants. Everything that goes into the tub comes out of the tub in a homogeneous, but uniform, mix. That may be just peachy for wood waste processing, but not so good for a mixed waste stream. But with price tags in the hundreds of thousands of dollars, operations producing less than _____ cubic yards of finished compost a year may not like the cost-to-benefit ratio for these big machines and find contract grinding or buying wood chips and sawdust from other sources to be more economical than acquiring a mechanical behemoth. Blending. For the backyard, motorized versions of cranked and tumbled turners will do the trick. Increasing volumes demand the adoption of a backhoe or bobcat for feedstock blending. The most common mixing unit used by commercial composting operations is a batch or continuous feed auger system[5] where feedstocks are added to a hopper fitted with one or more large corkscrews that blend the material. With batch blending, a loader operator will select and move all raw materials to the blending hopper one bucket at a time, according to prescribed ratios. The admixture is not discharged from the hopper until all feedstocks have been added and blended to desired levels of homogeneity and porosity. In a continuous feed system, feedstock volumes and types are metered into the hopper manually or electronically as the auger simultaneously rotates and discharges blended admixture. A loader operator may still be filling feed hoppers, but the flow of material out of those hoppers and into the blending hopper is not impacted by this activity. It’s also possible to have feedstocks off-loaded directly into auger-equipped containers discharging directly to the feeder belts, eliminating the loader entirely. Proponents of batch blending say it results in a blend of higher quality. Those who favor continuous feed prefer the advantage of a smaller footprint, easier maintenance, and greater efficiencies over the batch system, but up-front costs are higher. There are other types of blending equipment used in composting, including plow blenders, ribbon blenders, and screw conveyors. Plow blenders are similar to pug mills and achieve similar results. A ribbon blender typically has both inner and outer ribbons of coiled metal attached to a horizontal shaft, moving materials in opposite directions. Screw conveyors mix feedstocks as they are transported from Point A to Point B, are used in continuous feed systems, and work on the same principle as an auger. The flexible screw has the ability to move materials horizontally, vertically, and around tight corners. Why would a composting system designer choose one mixing method over another? (add selection criteria) Placement and turning. The composting industry is no longer forced to modify equipment originally designed for the feed mill, feed lot, or rock quarry. Though enterprising composters still use snow-blowers, potato diggers, manure spreaders and dump trucks to build and turn windrows, such feats of engineering and ingenuity are now optional, not necessity. But specialized equipment costs money, and the small commercial or on-farm operation can manage and entire operation with just a bucket attachment for the tractor, scooping up a couple of cubic yards at a time from one pile and dropping it in another. At the same time, material is aerated and fluffed as it cascades out of the bucket and onto the pilee[6] A step up from the by-the-bucket method is to deposit the blended admixture into a dump bed (wagon, trailer, or truck) driven slowly through the field while tipped, dropping the material into a nice, even row. The settings on spreader trucks designed for land application can also be adjusted to create windrows. However, for all of these methods, the windrow will still need to be shaped and built up to the proper height with a bucket. Conveyor systems and large-capacity buckets on front end loaders will improve efficiencies for high volume operations, as will tractor-driven and self-propelled windrow turners. Processing. Turning is processing for windrow operations … no additional equipment needed. The low capital investment required for windrow is one of the reasons why it remains the dominant method of the industry. A rough cost for construction of a windrow composting facility is about $_____ per ton. Processing equipment for static forced aeration systems include the addition of an air delivery system consisting of piping and motor/blower units. Fans can be set for updraft or downdraft. The trick is to find a balance of velocity and volume that will keep oxygen supplies up and moderate heat without over drying. Construction of an outdoor static forced aeration facility is about $_____ per ton. Large in-vessel systems use highly-engineered equipment that may tumble, aerate, mist, and filter air within one compact unit. The construction price tag of an in-vessel facility, minus the building, is about $_____ per ton. Post-Screening. Trommels are used for finished compost, but star and deck screens are also popular choices. Star screens effectively break up clumps with rotating disks, but abrasive materials damage the disks, and star screens are best reserved for finished compost. Star screens are more complicated than trommels and more expensive to operate. A deck screen acts like a giant sifter, with a metered flow of material from a hopper dropping onto a vibrating box of wire mesh. The deck box may be single or multi-layered and set on an angle. This allows the finer material to filter through the mesh and the larger particles to vibrate off into another pile. Grinding/screening equipment and best uses
[1] SOURCEE: The On-Farm Composting Handbook (NRAES-54), www.nraes.orgg [2] Use only wet weight or only dry weight, not both, when doing calculations. When looking for a used copy of this book on the web, we found only two … a testament to the publication’s reputation as a bookshelf staple. [3] Websites like www.organicgardening.com offer carbon and nitrogen content of common compost feedstocks as a percentage, making a rapid calculation of ratios possible. Multiplying total feedstock weight in pounds by percent of carbon and nitrogen content results in the C:N ratio. Using 100 pounds of material at 40 percent carbon content and one percent nitrogen content as an example: 100 ÷ 40% = 40 pounds C and 100 ÷ 1% = 1 pound N equals a C:N ratio of 40:1. [4] Google™ for compost mix calculator. The Klickitat County, Washington, website provides one that’s easy to use. Select feedstocks from drop-down menus, add the cubic feet available, and it tells you how many parts of each to combine to achieve a desirable C:N ratio … 12 parts swine manure mixed with 50 parts soft wood chips results in a C:N ratio of 31:1, by the way. [5] Such blending units may be referred to as a pug mill. The term pug mill was originally associated with refining and mixing clay or pugging, the practice placing clay or mortar between the joists under floor boards to deaden sound. Today, it’s commonly used to describe any mixing or blending unit equipped with rotating shafts or paddles [6] The bucket should be suspended a foot or so above the apex of the pile.
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